Old phone is still connected..?

I gave my old iphone to my dad after logging out of everything and deleting all my old stuff. I use a new iphone and my icloud is logged in here. My dad also has a seperate icloud account. However when he saves someone to his contacts it also gets saved to mine, same with when I save someone new. Also my he receives my calendar notifications as well. I checked setting and it showed different apple ID and i cloud account. What might be the reason and how can I solve the issue?